Fixed-wing aircraft landing is accomplished by reducing the airspeed and the rate of decent (e.g., sink rate) such that the aircraft descends at a suitably low rate to allow for a gentle touchdown. Reduction of airspeed and rate of decent are accomplished by reducing thrust and/or inducing a greater amount of drag on the aircraft (e.g., by using flaps, landing gear, speed breaks or the like). When a fixed-wing aircraft approaches the ground, the aircraft executes a flare or roundout (commonly known as flaring or a flaring maneuver). Flaring is a smooth transition from a normal approach attitude to a landing attitude by increasing the angle of attack of the aircraft (e.g., the aircraft pitches upward). When the angle of attack is increased, lift is momentarily increased, thereby decreasing the rate of descent of the aircraft, allowing the aircraft to settle to an acceptable speed and touchdown on its landing gear.
Automatic landing systems are widely used to provide reliable and safe automatic or semi-automatic landings, particularly in low visibility conditions (e.g., landings in fog, in rain or at night or other landings under adverse weather conditions). In order to automatically land the aircraft, the aircraft must be accurately controlled to provide a smooth landing flare path as the aircraft approaches ground level.
In many instances, the terrain at ground level prior to a landing zone (e.g., a runway) is not always flat. Instead, the ground prior to a landing threshold (e.g., denoting the beginning and end of the designated space for landing) may have irregular topography. Thus, the majority of automatic landing systems utilize radio altitude (e.g., a measure of the height of the aircraft above ground level) to respond to topographical changes in terrain height as the aircraft passes over them. However, reliance on radio altitude can cause improper flare maneuvers due to undesirable pitch activity of the aircraft. Such undesirable pitch activity may often results in a touchdown at an inappropriate distance from the landing threshold (e.g., too far down the runway) or an inappropriately high sink rate of the aircraft. Undesirable pitch activity may further result in non-use or disconnection of the automatic landing system and/or abortion of the landing.
Accordingly, those skilled in the art continue with research and development efforts in the field of automatic landing systems for aircraft.